Surface polysaccharides of pathogenic bacteria, including capsular polysaccharides (CPS) or lipopolysaccharides (LPS), serve both as essential virulence factors and as protective antigens. The age-related and T-cell independent immunogenicity of CPS limit their use as vaccines especially in infants and young children. LPS are too toxic to be administered. Accordingly, their O-specific polysaccharide (O-SP), that share the virulence promoting and protectiveness of CPS, must be purified: O-SP are too small to be immunogenic (haptens). Covalent binding of CPS or of O-SP to medically-useful proteins to form conjugates both increases their immunogenicity and confers T-cell dependence to these saccharides. The O-SP of Shigella sonnei and of Shigella flexneri 2a were bound to bacterial toxoids. In adults and then in 4-7 year-olds, both conjugates were safe and induced statistically significant and long-lived rises of IgG antibodies to the homologous LPS. Similar, though lesser rises of IgM and IgA anti-LPS were also induced. Re-injection of S. flexneri 2a conjugate induced a booster response in the recruits and the 4-7 years old. A Phase 3 trial showed that one injection of S. sonnei O-SP, bound to a non-toxic recombinant Pseudomonas aeruginosa exoprotein A (rEPA) protected army recruits against outbreaks with this pathogen. Importantly, there was a statistically-significant correlation between the levels of serum IgG anti-LPS and the efficacy of the conjugate. Two methods were developed that increased the immunogenicity of the Shigella conjugates in mice: another carrier protein, a genetically-inactivated Corynebacterium diphtheriae toxin (CRM9) was a superior carrier for S. sonnei O-SP and treatment of rEPA with succinic anhydride, a non-toxic mild akylating agent that converts amino groups of proteins to carboxyls, increased the immunogenicity of S. flexneri 2a O-SP. A phase 1 study in adults of these Shigella conjugates confirmed their safety and immunogenicity; the improved immunogenicity was less marked than in mice. A phase 2 study in 1-4 years old showed an improved immunogenicity of the new S.flexneri 2a conjugate but lesser immunogenicity of the S.sonnei conjugate. A phase 3 study of the modified S.flexneri 2a and the original S. sonnei conjugates are in preparation. In collaboration with the Lanzhou Vaccine Institute and Provincial Medical Center in Henan, China, a clinical trial of these two conjugates is being planned. To investigate if concurrent administration of a cross-reacting along with a homologous CPS has an advantage over the use of the homologous CPS alone, the cell wall polysaccharide (PS) of Bacillus pumilus, SH18, reported to cross react with the CPS of haemophilus influenzae type b (Hib), was isolated by conventional methods and it?s structure investigated using GC-MS. It was shown to contain glycerol, ribitol and 2-acetamido-2-deoxyglucose in a molar ratio of 0.2:1.0:0.2 and 17% phosphate. Besides with the anti Hib it cross reacted with anti Staphilococcus epidermidis. Methods to prepare a conjugate of this PS are investigated. Neisseria meningitidis group A causes endemic and epidemic meningitis, notably in the meningitis belt of Africa. A CPS vaccine , effective and available, is underutilized. To further improve it?s immunogenicity, as was done for other CPS, methods of binding it to a carrier protein are being investigated. A double mutant of Bordetella pertussis, producing a genetically-inactivated toxin and deficient in FHA synthesis was developed. Effort is directed towards increasing production of this B. pertussis strain as a more easily purified pertussis toxin for a monocomponent vaccine and as a carrier protein for pneumococcal type 14 CPS. Clostridium difficile is a major cause of hospital-acquired diarrhea following antibiotic usage: the diarrhea is mediated by two exotoxins, A and B. Toxin A, considered to be the major toxin, in the extreme form will cause pseudomembranous colitis. A genetically-derived toxin mutant (rARU) induces both antitoxin and protects animals from infection with C. difficile. The succinylation of rARU improved its solubility and did not detectably affects its antigenicity. Techniques to prepare the mutant toxins A for clinical use have been worked out. Three polysaccharide of varying composition, pneumococcus type 14, Escherichia coli K1 and S. flexneri 2a were conjugated to succinylated rARU. The resultant conjugates induced high levels of both anti-polysaccharide and antitoxin. Preparation of toxin A conjugates for clinical evaluation is underway. Borrelia burgdorferi, a spirochete transmitted though the bite of infected Ixodes ticks, is the etiologic agent of Lyme disease. A protein vaccine against it is available but is not effective below the age of 12 years. LPS has been described in other spirochetes but it's presence in B. burgdorferi has been debated. So far we have not been able to confirm it's presence. The search for LPS revealed a unique glycolipid cosisting of glycerol and galactose as the carbohydrate moiety. There is evidence that this glycolipid is surface exposed Injected in complete Freund's adjuvant it induced specific antibodies.